Ventilation systems commonly used in buildings often include a ventilation duct to one end of which a fan is connected. A ventilation device is provided at the other end. One or several dampers and pressure equalizing boxes are arranged in the ventilation duct and in the ventilation system for regulating the air flow at various positions along the ventilation duct. The ventilation duct often extends over several different areas in the building, for the ventilation of the areas. The damper is adjustable in the ventilation duct, here referred to as the air flow orifice, with which the air flow through the ventilation device between the ventilation duct and outlying area can be regulated. As the damper is connected to the ventilation duct, the air flow in to or out from the ventilation duct is regulated by adjusting the size of the air flow orifice by adjusting the position of the damper in the ventilation duct.
The air flow through a ventilation system depends on factors such as fan power, the dimensions of the ventilation duct and damper position that adjusts the size of the air flow orifice. The dimension of the ventilation duct refers here to its cross sectional area. When the ventilation system comprises a plurality of ventilation devices and dampers, the latter are generally set so that the various ventilation devices and dampers have different sizes at the air flow orifice thereby to adjust the pressure equalizing in the ventilation system. By adjusting the air flow orifice of the various ventilation devices and the dampers, unnecessarily high pressures may be throttled away. This allows a predetermined air flow to be achieved by respective ventilation devices and dampers, i.e. that a desired degree of ventilation to be obtained in all the areas in which one or several ventilation devices are provided, or in different parts of the ventilation system. Too low air flow causes insufficient ventilation, while too high air flow causes increased energy costs.
The air flow, i.e. the amount of inlet air or exhaust air, is generally set by current practice, depending on the dimensions of the ventilation duct. To achieve this air flow, a certain pressure equalizing is needed in the ventilation system.
One problem with ventilation systems is that dirt is accumulated in the system and the system must be cleared/cleaned periodically to have a good air environment in the ventilated areas. In order to clear thoroughly a ventilation system it is necessary to remove dampers from the system to make it possible for the clearing tools to have excess to clear the system.
For these ventilation systems there are also rules prescribing that they have to be cleaned periodically, for example in Sweden there is a Mandatory Ventilation Control (OVK).
A further problem with ventilation systems is that they are difficult to install, as they are often located in places with difficult access and where the space is limited.
A further problem with ventilation systems is that the requirements of the tolerances of the parts of the system are high to achieve a ventilation system that is tight and efficient.
A further problem with ventilation systems is that there is a great pressure on prices, both in the manufacture of parts and the installation of the ventilation systems.
A further problem with these systems is that they produce noise that may be perceived as disturbing. Therefore, for these ventilation systems, there are limit values for a recommended maximum sound power level. Sound is especially produced in the ventilation devices at the air flow through the opening to the surroundings, i.e. the air flow orifice. The limit values for the allowed sound power level produced by respective ventilation devices and dampers limit how large pressure drop that can be achieved over the ventilation device or the damper, i.e. which degree of opening the respective ventilation device and the damper may have. This also sets limits to which air flow can be obtained through the ventilation system.
As mentioned above, a ventilation system usually contains a plurality of ventilation devices and dampers at different distances from the fan. In ventilation systems for inlet air, the pressure generated by the fan is lowest at the ventilation device that is positioned farthest from the fan, and thus this ventilation device is set to maximum orifice, i.e. that this ventilation device has a maximum size of the air flow orifice. With farthest means the ventilation device that has the lowest pressure drop. The pressure required for this ventilation device to provide a specified air flow determines the operational condition of the fan. To minimize energy consumption, the pressure drop should be as low as possible. In ventilation systems for exhaust air, the principle for the pressure is reversed.
At same time, a specified air flow must be obtained also at the other ventilation devices and dampers, which are closer to the fan, and thus experiencing a greater pressure from the fan in inlet air systems. Therefore, a certain degree of throttling of the pressure over respective damper is necessary so that the air flow will neither exceed nor fall below a specified air flow. However, the recommended highest sound power level sets limits to how much pressure across a damper can be throttled, because of the noises generated by the flow of air through the damper. As we will describe more in detail here below, factors such as a size of the air flow orifice of the damper, the dimensions of the damper and the size of an air flow through the same have an impact on the sound power level generated in the ventilation device by the air flow passing there through. Therefore, the degree of throttling of the pressure on a damper that as maximum can be achieved over a damper, without exceeding the recommended highest sound power level, should be the as high as possible to obtain an effective ventilation throughout the entire ventilation system. Globally, all these factors thus set limitation for the ventilation system.
Even if ventilation systems for inlet air ventilation have been described here above, the same applies also on exhaust air ventilation, though the pressures are reversed.